“Charles as a young man had an immense crush on Barbra Streisand,” he told INSIDE EDITION. “Even at Cambridge University he had a poster of Barbra Streisand on the wall of his room.”

In 1974, Charles even visited the set of the movie, Funny Girl, where he spent 20 minutes with the star. But Andersen claims it was another two decades before their first tryst.

“Flash forward 20 years, he's back in Los Angeles and they had a secret rendezvous at the Bel Air Hotel that no one knows about," Andersen said.

One person knew though, he added.

“Something was going on and Diana knew all about it,” he said.

The book also lays out the bitter rivalries between the powerful women in the British royal family. While it’s all smiles for the cameras, behind the scenes it’s more like “Game of Thrones,” according to Andersen, a journalist who has authored more than 30 books.

“Great wealth, power, secrets, tragedy, scandal – it's all there in the royal family,” he said.

And with the Queen celebrating her 90th birthday this week, Andersen says there’s a secret plan – codenamed, "London Bridge" – to allow Prince William to become king while he’s still relatively young.

“This master plan calls for the Queen to step aside,” he said. “Charles takes the throne and agrees to reign for 15 years. He steps aside and allows William and Kate to take the throne in their mid-forties.”